This application relates generally to spark plugs for internal combustion engines and, more particularly, to a jamb nut to insulator interface that to reduce loads on the spark plug insulator.
Traditional spark plug construction includes an annular metal casing having threads near one end and a ceramic insulator extending from the threaded end through the metal casing and beyond the opposite end. A central electrode is exposed near the threaded end and is electrically connected through the insulator interior to a terminal which extends from the opposite insulator end to which a spark plug ignition wire attaches.
The force applied to seal the spark plug in the head is the result of torque transmitted by the threaded metal casing; hence, the threaded portion of the metal casing must be sturdy and of substantial size. A portion of the metal casing is formed to be engaged by a socket tool to provide torque to the threaded portion. The threaded portion is located away from the portion which is engaged by the socket tool.
To facilitate the controlled and efficient exhaust of gases from a combustion chamber, the valves are sometimes increased in size. This may necessitate a decrease in the size of the spark plug, a reduction in the size and sturdiness of the threaded metal casing end, and, in particular, a decrease in the inside diameter of the metal bore of the spark plug and in the combustion chamber wall area available to threadedly receive the spark plug.
The maximum diameter of the ceramic insulator is in turn determined by the size of the hex or jamb nut portion of the outer shell. Accordingly and for smaller hexes the barrel diameter of the ceramic insulator is reduced. The diameter ceramic insulator is also dictated by the clearance needed for shell hex and crimp along with the shoulder needed to keep the ceramic in place during a lifetime of operation.
In the past and for 14 mm and 16 mm spark plug hexes (12 mm spark plugs), the diameter of the ceramic insulator is different for each application and requires different ignition boots. For example, the 14 mm hex uses a 9.0 mm diameter ceramic barrel and the 16 mm hex uses a 10.5 mm diameter ceramic barrel.
Accordingly, it would be desirable to provide a 14 mm hex spark plug that can use the larger diameter insulator of the 16 mm hex spark plug.
The decrease in the diameter of the spark plug may also reduce the spark plugs ability to hold onto its ground shield during removal. A higher strength steel jamb nut has been proposed to combat this problem however, a higher strength steel jamb nut is assembled to the insulator with higher loads than a lower strength steel jamb nut thus, the impact strength of the insulator may be negatively affected.
Accordingly, the inventor herein has recognized that it is desirable to provide a jamb nut to insulator interface that reduces loads upon the insulator. In addition, the inventor has recognized that it is also desirable to use smaller spark plug hex designs with ceramic insulator barrels and ignition wires typically associated with larger spark plug hex designs.